<!DOCTYPE html>
<html>
<head>
  <title>background-position: top, bottom reference</title>
  <style type="text/css">
#outer
{
  border: 1px solid black;
  width: 128px;
  height: 128px;
}
#inner1
{
  margin-left: 48px; /** 128px - 128px * 0.50 + 32px * 0.50 - 32px **/
  margin-top: 0px;
  width: 32px;
  height: 32px;
  background-image: url(aqua-32x32.png);
}
#inner2
{
  margin-left: 48px; /** 128px - 32px - 32px **/
  margin-top: 64px; /** 128px - 128px * 0.50 + 32px * 0.50 - 32px **/
  width: 32px;
  height: 32px;
  background-image: url(aqua-32x32.png);
}
  </style>
</head>
<body>
<div id="outer"><div id="inner1"></div><div id="inner2"></div></div>
</body>
</html>
